1. Field of the Invention
The present invention relates to a method of floatingly supporting a metallic strip in a direct firing type continuous heat treating furnace.
2. Description of the Prior Art
When a horizontally transported metallic strip of, for example, stainless steel is subjected to an annealing treatment, an apparatus for horizontally supporting the metallic strip is required. In a conventional heat treatment of a relatively thin metallic strip of aluminum, copper, or the like, a non-contact supporting method is employed. The reason for this is that the treating temperature is approximately 800.degree. C. and the material to be treated is relatively light.
In this method, furnace gas is circulated and supplied to floater nozzles disposed in a furnace using booster fans so that the metallic strip may be floatingly supported by the static pressure generated by pressurized gas from the nozzles. However, this method cannot be applied to a metallic strip of, for example, stainless steel. Since this kind of metallic strip is heavy and the treating temperature thereof is in the range of 1000.degree.-1200.degree. C., the metallic material of the booster fans must exhibit good resistance to high temperatures and the fans must rotate at a high speed. Such a material, however, is not available.
Accordingly, in a heat treatment of the metallic strip of stainless steel or the like, water-cooled support rolls of asbestos are generally employed as a support means.
In this heat treatment, however, the high temperature metallic strip is brought into direct contact with the support rolls, thereby occasionally causing the picking-up or dragging on the surface of the metallic strip or the damage of the rolls. Furthermore, since asbestos is restricted in its use, careful consideration has been given to the use of the floatingly supporting method in the case where a horizontally transported metallic strip of stainless steel or the like is subjected to a heat treatment.
In this method, since gas having a temperature greater than 900.degree. C. must be pressurized to a high pressure, booster fans would have to exhibit good resistance to high temperature and rotate at a high speed. If metallic booster fans are used, the temperature of booster gas is limited to a temperature below 800.degree. C. in view of the durability of the fans.
Furthermore, the pressure of the booster gas is limited to a pressure below 350 mmH.sub.2 O converted at the normal temperature. In this connection, when gas having a density equivalent to that of air at 1000.degree. C. is pressurized to 350 mmH.sub.2 O, the fans are required to have a capacity of 1470 mmH.sub.2 O at the normal temperature. This value requires a fan speed of over 2000 rpm. Since no metals can withstand such severe conditions, the use of ceramic fans has been proposed. However, the ceramic fans exhibit low resistance to vibration, thermal impulse and internal temperature difference and are therefore unsatisfactory for practical applications.
It is, therefore, quite difficult to apply the floatingly supporting method to a treatment at a high temperature of 1000.degree.-1200.degree. C., such as an annealing treatment for stainless steel.